Such loudspeakers comprising an acoustic radiator capable of supporting bending waves and a transducer mounted on the acoustic radiator to excite bending waves in the acoustic radiator to produce an acoustic output are described, for example, in WO97/09842 (incorporated herein by reference).
According to that document, the bulk properties of the acoustic radiator may be chosen to distribute the resonant ending wave modes substantially evenly in frequency. In other words, the bulk properties or parameters, e.g. size, thickness, shape, material etc., of the acoustic radiator may be chosen to smooth peaks in the frequency response caused by “bunching” or clustering of the modes. The resultant distribution of resonant bending wave modes may thus be such that there are substantially minimal clusterings and disparities of spacing. For panels of rectangular shape and isotropic bending stiffness, the document identifies particularly useful aspect ratios for the side dimensions, e.g. 1.134:1.
The transducer location may be chosen to couple substantially evenly to the resonant bending wave modes and, in particular, to lower frequency resonant bending wave modes. To this end, the transducer may be at a location where the number of vibrationally active resonance anti-nodes is relatively high and conversely the number of resonance nodes is relatively low. In the case of a rectangle, specific locations found suitable are at 3/7, 4/9 or 5/13 of the distance along the axes.
Analysis as taught in WO97/09842 leads not only to preferred locations for transducer means but also to the capability to identify actual locations where any selective damping should be applied to deal with any particular undesired frequency or frequencies. WO99/02012 similarly discloses the use of mass loading at localised positions. Both disclosures address the problem of certain frequencies that are dominant (having greater than average amplitude ratios that ‘stick out’) and thus distort the overall frequency response of a corresponding loudspeaker.
WO00/22877 discloses the selective local positioning of masses, e.g. in the range from about 2 to 12 grams, bonded to a bending wave panel to optimally tune the coupled resonances such that the overall response is suitably tailored. This technique has the specific advantage of extending the low frequency range of the assembly.
U.S. Pat. No. 5,615,275 describes a loudspeaker including a planar diaphragm that mounted in a frame and that is coupled at its rear surface to a speaker voice coil such that the voice coil acts like a piston, pressing on the rear surface of the diaphragm and causing sufficient vibration of the diaphragm to efficiently produce sound. Masses are resiliently mounted on the diaphragm so as to improve its frequency response characteristic, the number, size and precise positioning of the weights for any particular diaphragm being determined empirically. The weights act to neutralize or counter uncontrolled movement of the diaphragm at certain frequencies.
The present invention is specific to distributed resonant mode devices and has as an objective an improvement in the uniformity of distribution of resonant modes of such devices. As will be appreciated from the aforementioned WO97/09842, an increase in the uniformity of distribution of the resonant modes that underpin the operation of this genre of device will result in an improvement of the frequency response of the device itself. This may be particularly appropriate when, due to styling considerations or the need to fit a panel in an existing space, the preferred panel dimensions discussed above are not possible.